在System.Xml.XPath中启用XPath2查询（XPathException：无效令牌）

Question

Microsofts System.Xml.XPath nuget-package，可用于.NET 4.6，声称支持XPath 1.0和2.0。 De documentation说descibes命名空间：

System.Xml.XPath命名空间包含定义用于浏览和编辑XML信息项的游标模型的类作为XQuery 1.0和XPath 2.0数据模型的实例。

升级Visual Studio后，升级和我的所有项目到框架版本4.6我仍然无法获得最简单的XPath-2.0 for-expression工作。根据specification，他们应该工作。

我无法想象微软声称支持它实际上不支持的东西，所以显然我做错了什么。 如何正确使用XPath2查询？

[TestMethod] 
public void TestXPath2() 
{ 
    // The System.Xml.XPath namespace contains the classes that define a cursor model for navigating and editing XML information items as instances of the 
    // XQuery 1.0 and XPath 2.0 Data Model. 

    var expression = "for $x in /Root/Foo/Bar return $x"; 
    var compiledExpression = System.Xml.XPath.XPathExpression.Compile(expression); 
    // throws XPathException: "for ... has an invalid token" 
} 

P.S. 其实我是想什么，是为了得到这样的工作：

[TestMethod] 
    public void TestLibraryForCustomer1() 
    { 
     string xmlFromMessage = @"<Library> 
      <Writer ID=""writer1""><Name>Shakespeare</Name></Writer> 
      <Writer ID=""writer2""><Name>Tolkien</Name></Writer> 
      <Book><WriterRef REFID=""writer1"" /><Title>King Lear</Title></Book> 
      <Book><WriterRef REFID=""writer2"" /><Title>The Hobbit</Title></Book> 
      <Book><WriterRef REFID=""writer2"" /><Title>Lord of the Rings</Title></Book> 
      </Library>"; 

     var titleXPathFromConfigurationFile = "./Title"; 
     var writerXPathFromConfigurationFile = "for $curr in . return /Library/Writer[@ID=$curr/WriterRef/@REFID]/Name"; 

     var library = ExtractBooks(xmlFromMessage, titleXPathFromConfigurationFile, writerXPathFromConfigurationFile).ToDictionary(b => b.Key, b => b.Value); 

     Assert.AreEqual("Shakespeare", library["King Lear"]); 
     Assert.AreEqual("Tolkien", library["The Hobbit"]); 
     Assert.AreEqual("Tolkien", library["Lord of the Rings"]); 
    } 

    [TestMethod] 
    public void TestLibraryForCustomer2() 
    { 
     string xmlFromMessage = @"<Library> 
       <Writer ID=""writer1""> 
        <Name>Shakespeare</Name> 
        <Book><Title>Sonnet 18</Title></Book> 
       </Writer> 
       <Writer ID=""writer2""> 
        <Name>Tolkien</Name> 
        <Book><Title>The Hobbit</Title></Book> 
        <Book><Title>Lord of the Rings</Title></Book> 
       </Writer> 
      </Library>"; 

     var titleXPathFromConfigurationFile = "./Title"; 
     var writerXPathFromConfigurationFile = "../Name"; 

     var library = ExtractBooks(xmlFromMessage, titleXPathFromConfigurationFile, writerXPathFromConfigurationFile).ToDictionary(b => b.Key, b => b.Value); 

     Assert.AreEqual("Shakespeare", library["Sonnet 18"]); 
     Assert.AreEqual("Tolkien", library["The Hobbit"]); 
     Assert.AreEqual("Tolkien", library["Lord of the Rings"]); 
    } 


    public IEnumerable<KeyValuePair<string,string>> ExtractBooks(string xml, string titleXPath, string writerXPath) 
    { 
     var library = XDocument.Parse(xml); 
     foreach(var book in library.Descendants().Where(d => d.Name == "Book")) 
     { 
      var title = book.XPathSelectElement(titleXPath).Value; 
      var writer = book.XPathSelectElement(writerXPath).Value; 
      yield return new KeyValuePair<string, string>(title, writer); 
     } 
    } 

Answer 1

我怀疑的XPath 2.0查询语言真正支持。

但是对于像您的要求那样简单的事情，XPath 1.0单行就足够了。

string xml = @"<Root> 
    <Foo ID=""foo1"">One</Foo> 
    <Foo ID=""foo2"">Two</Foo> 
    <Bar><FooRef REFID=""foo2"" /></Bar> 
    </Root>"; 
var doc = XDocument.Parse(xml); 
var matchingFoo = doc.XPathSelectElement("/Root/Foo[@ID = //Bar/FooRef/@REFID]"); 

Assert.AreEqual("Two", matchingFoo.value); 

对于超出XPath 1.0功能的查询，请尝试使用LINQ。

在您的扩展示例中，您想要将我们链接到书籍的作者。这是很容易在LINQ做加盟，这样的：

var xmlFromMessage = @"<Library> 
    <Writer ID=""writer1""><Name>Shakespeare</Name></Writer> 
    <Writer ID=""writer2""><Name>Tolkien</Name></Writer> 
    <Book><WriterRef REFID=""writer1"" /><Title>King Lear</Title></Book> 
    <Book><WriterRef REFID=""writer2"" /><Title>The Hobbit</Title></Book> 
    <Book><WriterRef REFID=""writer2"" /><Title>Lord of the Rings</Title></Book> 
    </Library>"; 

var libraryDoc = XDocument.Parse(xmlFromMessage); 

var library = from title in libraryDoc.Descendants("Title") 
    join writer in libraryDoc.Descendants("Writer") 
    on title.Parent.Element("WriterRef").Attribute("REFID").Value equals writer.Attribute("ID").Value 
    select new KeyValuePair<string, string>(title.Value, writer.Value); 

现在library是IEnumerable<KeyValuePair<string,string>>与标题/写器配对。

Answer 2

Tomalek指出正确：

• 的XPath 2.0未在.NET支持（在System.Xml.XPath），周期
• 的数据模型和查询语言是分开的东西。

所以我解决了它通过使用第三方XPath 2库，XPath2 nuget package。这允许表达式如

for $c in . return ../Writer[@ID=$c/WriterRef/@REFID]/Name 

请注意，我需要使用从书到作家的相对路径。这确实不工作：

# does not work due to the absolute path 
for $c in . return /Library/Writer[@ID=$c/WriterRef/@REFID]/Name 

以供将来参考：安装的NuGet pacage后，此代码的工作：

using Microsoft.VisualStudio.TestTools.UnitTesting; 
using System.Collections.Generic; 
using System.Linq; 
using System.Xml.Linq; 
using Wmhelp.XPath2; 

namespace My.Library 
{ 
    [TestClass] 
    public class WmhelpTests 
    { 
     [TestMethod] 
     public void LibraryTest() 
     { 
      string xmlFromMessage = @"<Library> 
       <Writer ID=""writer1""><Name>Shakespeare</Name></Writer> 
       <Writer ID=""writer2""><Name>Tolkien</Name></Writer> 
       <Book><WriterRef REFID=""writer1"" /><Title>King Lear</Title></Book> 
       <Book><WriterRef REFID=""writer2"" /><Title>The Hobbit</Title></Book> 
       <Book><WriterRef REFID=""writer2"" /><Title>Lord of the Rings</Title></Book> 
      </Library>"; 

      var titleXPathFromConfigurationFile = "./Title"; 
      var writerXPathFromConfigurationFile = "for $curr in . return ../Writer[@ID=$curr/WriterRef/@REFID]/Name"; 

      var library = ExtractBooks(xmlFromMessage, titleXPathFromConfigurationFile, writerXPathFromConfigurationFile).ToDictionary(b => b.Key, b => b.Value); 

      Assert.AreEqual("Shakespeare", library["King Lear"]); 
      Assert.AreEqual("Tolkien", library["The Hobbit"]); 
      Assert.AreEqual("Tolkien", library["Lord of the Rings"]); 
     } 


     [TestMethod] 
     public void TestLibraryForCustomer2() 
     { 
      string xmlFromMessage = @"<Library> 
       <Writer ID=""writer1""> 
        <Name>Shakespeare</Name> 
        <Book><Title>Sonnet 18</Title></Book> 
       </Writer> 
       <Writer ID=""writer2""> 
        <Name>Tolkien</Name> 
        <Book><Title>The Hobbit</Title></Book> 
        <Book><Title>Lord of the Rings</Title></Book> 
       </Writer> 
      </Library>"; 

      var titleXPathFromConfigurationFile = "./Title"; 
      var writerXPathFromConfigurationFile = "../Name"; 

      var library = ExtractBooks(xmlFromMessage, titleXPathFromConfigurationFile, writerXPathFromConfigurationFile).ToDictionary(b => b.Key, b => b.Value); 

      Assert.AreEqual("Shakespeare", library["Sonnet 18"]); 
      Assert.AreEqual("Tolkien", library["The Hobbit"]); 
      Assert.AreEqual("Tolkien", library["Lord of the Rings"]); 
     } 


     public IEnumerable<KeyValuePair<string, string>> ExtractBooks(string xml, string titleXPath, string writerXPath) 
     { 
      var library = XDocument.Parse(xml); 
      foreach (var book in library.Descendants().Where(d => d.Name == "Book")) 
      { 
       var title = book.XPath2SelectElement(titleXPath).Value; 
       var writer = book.XPath2SelectElement(writerXPath).Value; 
       yield return new KeyValuePair<string, string>(title, writer); 
      } 
     } 
    } 
}